The present invention relates to a mass spectrometer and a method of mass spectrometry.
In drug metabolism studies metabolites of interest cannot usually be predicted. This is because the formation of metabolites may be determined by novel enzymatic reactions and by factors which are difficult to predict in advance such as bio-availability.
At present in order to detect and identify metabolites it is known to separate out the many different components present, in a complex biological matrix using liquid chromatography (LC or HPLC). The mass or mass to charge ratio of the components eluting from the liquid chromatograph is then measured using mass spectrometry (MS).
It is usually necessary to make many measurements using LC-MS. (wherein parent, ions eluting from a liquid chromatograph are mass analysed) and LC-MS-MS (wherein specific parent ions eluting from a liquid chromatograph are fragmented and the fragment products are mass analysed) often in both positive and negative ionisation modes. The exact accurate mass or mass to charge ratio of the components eluting from the liquid chromatograph is normally determined since this enables many of the large number of endogenous peaks present in different biological matrices such as bile, plasma, faeces and urine to be discounted.
Ions which are determined as having a mass to charge ratio which indicates that they may relate to a metabolite of interest are then fragmented in a collision cell. The resulting fragment products are then mass analysed enabling the structure of each possible metabolite to be predicted.
The conventional approach is, however, relatively time consuming since it is necessary to interrogate all of the mass spectral data to look for potential metabolites of interest. It is then necessary to arrange for all ions which are considered likely to relate to metabolites of interest then to be separately fragmented so that the structure of potential metabolites of interest can then be determined.
It will be appreciated that the process of searching mass spectra relating to a complex mixture, identifying potential ions which may relate to metabolites of interest, selecting certain ions to be fragmented, fragmenting the ions of interest and then mass analysing the fragment products can be relatively time consuming.
Within the pharmaceutical and biotechnology industries it is particularly important to be able to analyse samples quickly and accurately. This has led to automated methods wherein the major peaks present in a mass spectrum are automatically selected for analysis by MS/MS (wherein specific parent ions are selected for fragmentation). This allows the user to acquire parent ion mass spectra and several MS/MS spectra from a single HPLC injection. It is known for to automatically select most intense peaks (i.e. ions) in a parent ion mass spectrum for subsequent analysis by MS/MS. Some conventional systems allow a few filters to be defined to make this process slightly more efficient. For example, ions having certain masses or mass to charge ratios may be entered into a data system so that, they are automatically excluded from consideration. These masses or mass to charge ratios may, for example, correspond to the masses or mass to charge ratios of solvent peaks which are known to be present, or the masses or mass to charge ratios of components which have already been analysed.
An advantage of the conventional automated mode of data acquisition is that a fair degree of data may be acquired from a single HPLC injection. However, a disadvantage of the conventional approach is that only chose peaks which have an intensity which exceeds a pre-defined intensity threshold are normally selected for subsequent MS/MS analysis (i.e. fragmentation analysis). Importantly, if a large number of intense peaks are present or observed at any one particular time, then some of these peaks may simply fail to be selected for MS/MS analysis due to there being insufficient time to record all the separate MS/MS spectra within the relatively short duration of an observed chromatography peak.
Another particular problem with the conventional approach is that since the mass or mass to charge ratios of potential metabolites is not generally known in advance, then time can be wasted analysing a large number of peaks all or many of which subsequently turn out to be of little or no interest. This can also mean that actual peaks of potential interest which could have been analysed if only they had been recognised fail to be analysed at all because the mass spectrometer is busy analysing other ions.
It is therefore desired to provide an improved method of mass spectrometry and in particular to improve upon the current approach of searching for metabolites of interest.